Amorphous optical coupling structure for an electromagnetic wave detector and associated detector

ABSTRACT

The invention relates to an optical coupling structure intended to couple electromagnetic radiation to the surface of a photodetector, wherein a coupling surface paved along mutually perpendicular first and second directions by a set of N series (M1i, M2i, . . . . Mni) of first features, second features, . . . nth features, the features being identical within any one series, the features being distributed along the first and second directions, the distance between the centers of two adjacent features or the inter-reticular distances between two adjacent features being variable. The subject of the invention is also a detector or a laser source comprising said coupling structure.

FIELD OF THE INVENTION

The field of the invention is that of electromagnetic wave detectorsmade of semiconductor material and especially one having a multiplequantum well structure, particularly one suitable for the infraredrange.

BACKGROUND OF THE INVENTION

The rapid advances made in epitaxial growth on GaAs-type substrates hasled to the development of a new class of electromagnetic wave detectorsusing the absorption of radiation around a wavelength λ corresponding tothe electron transition between various energy levels within one and thesame band or between the valence band and the conduction band. Thediagram shown in FIG. 1 illustrates this type of transition.

The recent evolution in performance of this type of component is due inparticular to the relatively easy production of semiconductorheterojunction multilayers in the standard system by MBE (molecular beamepitaxy), that is to say the GaAs/Ga_((1-x))Al_(x)As. By adjusting thegrowth parameters, the thickness of the quantum wells and the percentagex of aluminum in the barriers imposing the confinement potential, it ispossible to choose a narrow detection band (about 1 micron) centered ona given wavelength.

This type of structure has the advantage of providing very goodsensitivity because of the discretization of the energy levels withinthe conduction bands of the photoconductor materials used.

Within the context of intersubband transitions, so that this type oftransition is possible, it is necessary for the electric field of theincident electromagnetic wave to have a component along the direction ofgrowth of the layers, i.e. along the direction D indicated in FIG. 1,this direction being perpendicular to the plane of the layers.

It has already been proposed to use coupling means of the diffractiongrating type (cf. Goossen and Lyon, APL (1985), pp 1257-1259) forgenerating said perpendicular component, creating diffracted rays,especially lamellar (1D) gratings or steps for coupling only a singlepolarization of the light. However, crossed diffraction gratings arealso known for coupling the various electric field components of anincident ray, such as the laser source LS as illustrated in FIG. 2. Thematrix grating Rij diffracts the incident ray along both the directionDx and the direction Dy.

The major drawback of the use of gratings is the wavelength and angularresonance associated with the increase in absorption, thereby limitingthe use of these devices to a very narrow absorption window. Theseresonances are directly related to the periodic nature of the gratings.Thus, if it is desired to have a detector capable of detecting a rangeof wavelengths having a broader spectral band, solutions other thangrating structures have to be sought.

SUMMARY OF THE INVENTION

This is why the present invention proposes a novel amorphous opticalcoupling structure designed to couple electromagnetic radiation on thesurface of a photodetector, in order to eliminate the periodic effectswhile still ensuring effective optical coupling.

This is a structure in which a short-range order may be defined, whichappears in the Fourier components at spatial frequencies correspondingto the wavelengths at which the detector intended to use this couplingstructure is sensitive, but without being able to define a long-rangeorder corresponding to a periodic structure.

More precisely, the subject of the present invention is an opticalcoupling structure intended to couple electromagnetic radiation to thesurface of a photodetector, characterized in that it comprises acoupling surface paved along mutually perpendicular first and seconddirections by a set of N series of first features, second features, . .. nth features, the features being identical within any one series, thefeatures being distributed along the first and second directions, thecenters between two adjacent features or the inter-reticular distancebetween two adjacent features being variable and the first, second, . .. nth features being of square shape and/or of rectangular shape.

Advantageously, the density of features on the coupling surface isapproximately constant over the entire said surface.

Advantageously, the optical coupling surface consists of a set of Nseries of first, second, . . . nth identical elementary cells within thesame series constituting the paving, each first, second, . . . nthelementary cell comprising a feature homothetic with said elementarycell.

Advantageously, the average of the distances between the centers ofadjacent features or the average of the inter-reticular distancesbetween two adjacent features along the first direction and the averagealong the second direction are substantially equal to the wavelength ofthe electromagnetic radiation in the detector medium.

According to a first embodiment of the invention, the inter-reticularspace between the features is constant.

According to a second embodiment of the invention, each feature iscentered within an elementary cell, the inter-reticular distance betweenfeatures not being constant and the fill factor of the elementary cellsby the features being constant.

Typically, the coupling surface may comprise first, second, third andfourth features having dimensions of axa, bxb, axb and bxa,respectively.

The features may be equally well etched level with the coupling surfaceas produced on the surface of the coupling surface by conventionalphotolithography processes and they typically have an etch depth ofaround λ/4.

According to one embodiment of the invention, the paving may be obtainedby depositing a highly conductive layer of the gold or silver type.

The subject of the invention is also an electromagnetic wave detectorcomprising a multiple quantum well structure operating on interband orintersubband transitions by absorption of radiation around a wavelengthλ, and comprising optical coupling means for coupling said radiation,characterized in that the optical coupling means comprise an opticalcoupling structure as claimed in the invention.

In general, the object of the invention is to reinforce theelectromagnetic field in the form of optical modes within the activelayer and therefore the invention can be applied to intersubband orinterband transitions.

Advantageously, the detector may comprise a multilayer stack produced onthe surface of a substrate, said stack comprising the multiple quantumwell structure and external layers, the first and second features beingetched within one external layer.

The subject of the invention is also a matrix electromagnetic wavedetector, characterized in that it comprises a matrix of individualdetector elements as claimed in the invention, each individual detectorelement having a multilayer stack, said stack comprising the multiplequantum well structure and external layers, the first and secondfeatures being etched within one external layer, said elements beingproduced on the surface of a common substrate.

According to one embodiment of the invention, the stack of active layersis a stack of semiconductor layers of the doped GaAs or GaAlAs type, thesubstrate being of the doped or undoped GaAs type.

According to one embodiment of the invention, the detector may comprisea substrate that is transparent at the wavelength of the radiation and alayer that is reflective at said wavelength, said reflective layer beingon the surface of the features, so as to make the detector operate inreflection.

Finally, the subject of the invention is a laser source comprising amultiple quantum well structure operating on interband or intersubbandtransitions at a wavelength λ and comprising an optical couplingstructure according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood and other advantages willbecome apparent on reading the description that follows and thanks tothe appended figures among which:

FIG. 1 shows schematically a multiple quantum well structure accordingto the known art;

FIG. 2 illustrates a laser source and multiple quantum well detectorpossessing optical coupling means of the matrix diffraction gratingtype, according to the prior art;

FIG. 3 illustrates a first embodiment of an optical coupling structureaccording to the invention;

FIG. 4 illustrates a second embodiment of an optical coupling structureaccording to the invention;

FIG. 5 illustrates a third embodiment of an optical coupling structureaccording to the invention;

FIG. 6 illustrates an example of a multiple quantum well detectoraccording to the invention, seen in cross section;

FIG. 7 illustrates an example of a matrix detector according to theinvention; and

FIG. 8 illustrates an exploded view of a matrix detector according tothe invention.

In general, the coupling structure according to the invention comprisesa set of paved features on a coupling surface, these features beingdistributed in two orthogonal directions Dx and Dy, the density offeatures over the entire surface being substantially constant.

To produce an amorphous coupling structure it is necessary to distributethe features in an aperiodic manner. To produce this condition, we willdescribe below various nonexhaustive alternatives that allow the desiredobjective to be achieved.

DETAILED DESCRIPTION OF THE EMBODIMENT First Embodiment

The coupling surface is made up of four series of elementary features,two series of square features having dimensions of c×c, d×d and twoseries of rectangular features having dimensions of c×d and d×c, asillustrated in FIG. 3. The distance between features is constant andequal to e.

The length c+e is matched to a first wavelength λ₁ and the length d+e ismatched to a second wavelength λ₂ so as to obtain an effective couplingstructure over a certain spectral band, that is to say c+e≅λ₁/n andd+e≅λ₂/n, where n is the optical index of the detector medium.

Second Embodiment

The optical coupling surface is defined in terms of elementary surfaces,again called elementary cells having dimensions of a×a, a×b, b×a andb×b, as illustrated in FIG. 4. In this embodiment, four series offeatures (M1i, M2i, M3i and M4i) homothetic with said elementary cellsare employed. The features have dimensions of c×c, c_(M)×d_(M),d_(m)×c_(M) and d×d, respectively. The dimensions c_(M) and d_(M) aredetermined so as to maintain the same fill factor as for the a×a and b×bcells.

The length a is matched to a first wavelength λ₁ and the length b ismatched to a second wavelength λ₂ so as to obtain an effective couplingstructure over a certain spectral band. For each type of cell, thefeature is placed at the center of the cell, the spacing between eachfeature (e_(c)+e_(M), e_(M)+e_(d), e_(d)+e_(d), etc.) being variablewithin the structure.

Third Embodiment

The optical coupling surface is defined in terms of elementary surfacesagain called elementary cells having dimensions of a×a, a×b, b×a andb×b, as illustrated in FIG. 5.

In this embodiment, four series of features (M1i, M2i, M3i and M4i)homothetic with said elementary cells are employed. The features havedimensions of c×c, c×d, d×c and d×d, respectively.

The length c is matched to a first wavelength λ₁ and the length d ismatched to a second wavelength λ₂ so as to obtain an effective couplingstructure over a certain spectral band.

The fill factor is not the same between the square cells and therectangular cells.

In general, the optical structure according to the invention maycomprise etched features (the preferred embodiment as technologicallythis is the easiest to produce).

The detector may be conventionally produced on the surface of asubstrate S made of a possibly undoped semiconductor material. Anassembly of layers constituting an ohmic contact, called the lowercontact C₁ made of highly doped semiconductor material is deposited onthe surface of the substrate. This ohmic contact supports the set ofsemiconductor layers constituting the multiple quantum well structureMQW, this structure being in contact with an assembly of layersconstituting an ohmic contact called the upper contact C_(u), thedetection taking place between the two ohmic contact layers.Advantageously, the features may be etched in the ohmic contact layerC_(u) as illustrated in FIG. 6, which shows a cross-sectional view.

The above description has shown optical coupling configurations for anelementary detector that may advantageously be applied within thecontext of a matrix detector comprising individual elements, each ofthese individual elements having, on the surface, optical coupling meanscomprising diffraction features along the directions Dx and Dy.

FIGS. 7 and 8 illustrate an example of a matrix detector according tothe invention in which the set of features is produced on the surface ofa common substrate with an ohmic contact layer that is also common.

To produce this architecture, the procedure is as follows:

-   -   a first ohmic contact layer C₁ is produced on a substrate that        is transparent to the wavelengths at which the detector is        sensitive, said ohmic contact layer C₁ also being transparent;    -   a stack of layers constituting the multiple quantum well        structure is produced on this ohmic contact layer;    -   the second ohmic contact layer C_(u) is deposited;    -   the features are etched in the layer C_(u);    -   the etched portions have a thickness d₁, which is around λ/4;    -   the individual detection elements are defined by etching all of        the layers down to the surface of the lower contact layer C₁;    -   a reflective layer RL is deposited over the second ohmic contact        layer C_(u); and    -   advantageously, an encapsulation layer may be deposited on the        matrix detector thus obtained.

Embodiment Example

We will now describe an example of a detector according to the inventionthat operates in the infrared range, and more particularly one suitablefor the 8-12 micron range.

The lower ohmic contact layer made of Si-doped GaAs with a dopingcontent of 5×10¹⁸ cm⁻³ and a thickness of typically 2 microns isdeposited on an intrinsically undoped GaAs substrate.

The multiple quantum well structure is produced by the stacking of 50periods composed of an Si-doped GaAs layer with a charge carrierconcentration of 5×10¹⁸ cm⁻³ with a thickness of 5 nm, this beinginserted between two barrier layers consisting of Ga_(0.75)Al_(0.25)Aswith a thickness of 50 nm.

The upper contact layer is identical to the lower contact layer and alsohas a thickness of 2 microns.

The features of the amorphous coupling feature are produced within thisupper contact layer.

To obtain the desired diffracting effects at an operating wavelengtharound 9 microns, the etch depths are 1.2 microns and the spacings a andb of the features are 2.4 microns and 2.7 microns (the mean opticalindex of the structure being from 3.3 to 9 microns). The fill factor ofthe surface of the upper contact layer is typically around 50%.

1. An optical coupling structure intended to couple electromagneticradiation to the surface of a photodetector, comprising: an ohmiccontact layer having an optical coupling surface comprising a set of Nseries of first, second, . . . nth identical elementary cells within thesame series, each first, second, . . . nth elementary cell, wherein foreach type cell, a raised feature is formed at the center of the cell andextends outward therefrom, and an etched distance between each saidfeature is variable within the structure, and wherein the ohmic contactlayer has a thickness of about 2 microns and an etch depth of about 1.2microns; wherein the features comprise first features, second features,. . . nth features, the features being identical within any one series,the features being distributed along mutually perpendicular first andsecond directions, the etched distance between the centers of twoadjacent features or the inter-reticular distance etched between twoadjacent features being variable and the first, second, . . . nthfeatures being of square shape and/or of rectangular shape; wherein saidfeatures in said first direction are responsive to multiple wavelengthssuch that the periodic effects in said photodetector are eliminatedwhile the range of detectable wavelengths is expanded to a broaderspectral band; and wherein said features in said second direction areresponsive to multiple wavelengths such that the periodic effects insaid photodetector are eliminated while the range of detectablewavelengths is expanded to a broader spectral band.
 2. The opticalcoupling structure as claimed in claim 1, wherein the average of theinter-reticular distances between two adjacent features along the firstdirection and the average along the second direction are substantiallyequal to the wavelength of the electromagnetic radiation in the detectormedium.
 3. The optical coupling structure as claimed in claim 2, whereinthe features of square or rectangular shape have constantinter-reticular distances.
 4. The optical coupling structure as claimedin claim 3, wherein the coupling surface consists of four series ofelementary cells a×a, b×b, a×b, b×a.
 5. The optical coupling structureas claimed in claim 4, wherein the fill factor of the cells with thefeatures is constant, the distances between features being variable. 6.The optical coupling structure as claimed in claim 4, wherein the cellfill factor is variable.
 7. An electromagnetic wave detector comprisinga multiple quantum well structure operating on interband or intersubbandtransitions by absorption of radiation at a wavelength λ, and comprisingoptical coupling means for coupling said radiation, wherein the opticalcoupling means comprise an optical coupling structure as claimed inclaim
 1. 8. The electromagnetic wave detector as claimed in claim 7,wherein it comprises a multilayer stack produced on the surface of asubstrate, said stack comprising the multiple quantum well structure andexternal layers, the features being etched within one external layer. 9.The electromagnetic wave detector as claimed in claim 8, wherein thethickness of the first and second features is around λ/4.
 10. Theelectromagnetic wave detector as claimed in claim 8, wherein the stackof active layers is a stack of semiconductor layers of the doped GaAs orGaAlAs type, the substrate being of the doped or undoped GaAs type. 11.The electromagnetic wave detector as claimed in claim 7, wherein itcomprises a substrate that is transparent at the wavelength of theradiation and a layer that is reflective at said wavelength, saidreflective layer being on the surface of the features, so as to make thedetector operate in reflection.
 12. A matrix electromagnetic wavedetector, wherein it comprises a matrix of individual detector elementsas claimed in claim 7, each individual detector element having amultilayer stack, said stack comprising the multiple quantum wellstructure and external layers, the features being etched within oneexternal layer, said elements being produced on the surface of a commonsubstrate.
 13. A laser source comprising a multiple quantum wellstructure operating on interband or intersubband transitions at awavelength λ, and comprising optical coupling means for coupling saidradiation, characterized in that the optical coupling means comprise anoptical coupling structure as claimed in claim
 1. 14. Theelectromagnetic wave detector as claimed in claim 9, wherein the stackof active layers is a stack of semiconductor layers of the doped GaAs orGaAlAs type, the substrate being of the doped or undoped GaAs type. 15.The electromagnetic wave detector as claimed in claim 8, wherein itcomprises a substrate that is transparent at the wavelength of theradiation and a layer that is reflective at said wavelength, saidreflective layer being on the surface of the features, so as to make thedetector operate in reflection.
 16. The electromagnetic wave detector asclaimed in claim 9, wherein it comprises a substrate that is transparentat the wavelength of the radiation and a layer that is reflective atsaid wavelength, said reflective layer being on the surface of thefeatures, so as to make the detector operate in reflection.
 17. A matrixelectromagnetic wave detector, wherein it comprises a matrix ofindividual detector elements as claimed in claim 8, each individualdetector element having a multilayer stack, said stack comprising themultiple quantum well structure and external layers, the features beingetched within one external layer, said elements being produced on thesurface of a common substrate.
 18. A matrix electromagnetic wavedetector, wherein it comprises a matrix of individual detector elementsas claimed in claim 9, each individual detector element having amultilayer stack, said stack comprising the multiple quantum wellstructure and external layers, the features being etched within oneexternal layer, said elements being produced on the surface of a commonsubstrate.
 19. A laser source comprising a multiple quantum wellstructure operating on interband or intersubband transitions at awavelength λ, and comprising optical coupling means for coupling saidradiation, wherein the optical coupling means comprise an opticalcoupling structure as claimed in claim
 2. 20. The optical couplingstructure as claimed in claim 4, wherein said length a is matched to afirst wavelength and said length b is matched to a second wavelength asto obtain an effective coupling structure a predetermined spectral band.21. The optical coupling structure as claimed in claim 4, wherein foreach type cell, said feature is placed at the center of the cell, andthe spacing between each said feature is variable within the structure.